The present invention relates to cutting inserts.
A cutting insert must be precisely positioned on a tool holder in order to accurately machine a workpiece and also to avoid setting up stresses that may damage the cutting insert and the associated clamping member.
There are several types of devices for clamping the insert in a pocket of the tool holder. In general, the principle of each type of device sets out to facilitate the operation of setting up and removing the cutting insert, while seeking to safeguard against any improper installation, i.e., to avoid clamping the insert in a wrong position.
The pocket has a generally flat bottom surface, which thus defines the operational plane of the cutting insert, forming a bearing surface on which the insert will initially be able to slide to abut on two alignment walls, respectively, lateral or longitudinal which define a corner which defines a functional position. It may however be provided for the rear wall to have two mutually inclined sections which together define a corner, with a re-entrant or salient interior angle.
One way of shifting the insert is to push it back into a rear corner bounded by the two alignment walls which will align it, using a sliding member bearing on the tool holder. For example, a “horizontal” screw is provided, i.e. with the body thereof housed in a threaded hole in the tool holder with an axis parallel to the bottom surface and consequently also to the upper face of the cutting insert, so that the head “skims over” the top of the cutting insert while being urged backwardly the direction of this rear corner so that the underside of the head, which forms a radial protrusion abuts against a front abutting surface of the insert, in other words directed away from the rear corner, thereby urging the insert into the rear corner. To further effectively push the insert right up against the bottom surface over which it has slid, the front surface is usually a ramped surface, which consequently introduces a component for pushing it firmly up against the bottom surface of the pocket.
Another conventional solution is to provide a “vertical” screw freely passing through a central hole connecting the top and bottom faces of the insert, so that the underside of the head thereof descends while bearing against a rear section of the countersunk mouth of the hole in order to exercise, by a rearwardly-directed wedging effect, a force urging it into the rear corner and clamping it against the bottom surface.
The manufacture of cutting inserts having a through hole for a positioning and clamping screw nevertheless raises problems of quality. Indeed, the cutting insert is formed by sintering from a powdered metal material that is poured into a mold to achieve the desired filling level corresponding to a thickness of the desired insert. To avoid having to drill the hole after sintering the practice is to introduce in its place in the powder, a conical headed calibrated rod which forms a reserve corresponding to a mouthpiece for receiving a clamping screw head.
As the head of the rod is driven under force into the powder, it creates a strong compaction of the powder grains in a virtual volume constituting the conical wall of the mouth of the hole. During sintering, this non-homogeneity of the density of the material leads to a lack of homogeneity of mechanical properties, such as coefficient of thermal expansion. Now, a cutting insert is subject to large amplitude thermal cycles during use. It can consequently happen that the wall of the hole splits, especially in the case of inserts of small size, since a proportionate reduction in the diameter of the clamping screw cannot be applied, as at least a certain thread pitch must be preserved to ensure the thread has an effective height and retains a certain stiffness.
Another problem is the fact that chips, which are sliding backwardly over a rake face forming a front edge of the upper face, originating from a leading cutting edge bounding this rake face, strike the top of the screw head and damage in particular the slot or equivalent recess for screwing.